We used statistical methods on rock properties derived from more than 480 wells to catalog shale velocity and density trends in different pressure regimes in the Gulf of Mexico and evaluated the reasons for their variations. A detailed evaluation of the density and velocity trends revealed that in the northern part of the Louisiana shelf, unloading is the major mechanism of overpressure. The onset of overpressure occurs at depths around 3000 m where temperatures are normally greater than 70°C. The relationship of the temperature gradient increase and the velocity decrease to the smectite-illite transformation allowed us to believe that inelastic unloading may be the major mechanism for overpressure in this region. On the other hand, in the southern part of the Louisiana shelf, abnormal pore pressure is often caused by compaction disequilibrium where the sediment section has a low sand percentage. In this type of pressure regime, velocity and density values cease to change at the onset of overpressure and essentially remain at the same value below the onset.